how to check if an input is an integer (including negative and decimal inputs)

I want to convert an input from Celsius to Fahrenheit but I don't understand how to accept negative and decimal inputs while also eliminating the non digit inputs like 'abc'.

temp = input('enter a temperature in C: ')

if temp.isdigit():
    temp = int(temp)
    print(f'{temp * 1.8 + 32} F')
else:
    print('not a temp')

You can do something like this. I dont know if its the best approach, but it should work

def is_valid_temperature(temperature):
    try:
        float(temperature)  # Convert the inputted temperature to a float
        return True
    except ValueError: # If its something like abc, this will catch the exception and return False
        return False

def convert_celsius_to_fahrenheit():
    temp = input('Enter a temperature in Celsius: ')
    
    if is_valid_temperature(temp):
        temp = float(temp)  # Now that we know that it's valid and it will not crash, we can convert the input to float.
        fahrenheit = temp * 1.8 + 32
        print(f'{temp}°C is equal to {fahrenheit}°F')
    else:
        print('Invalid input. Please enter a valid number.')

# Call the function with our program
convert_celsius_to_fahrenheit()

If you want, you can do an improvement: Do separation of concerns, by separating the user input from the convert_celsius_to_fahrenheit function.